Lecture 19Land grading –

Land levelling design methods - plane method

Land grading is reshaping of the field surface to a planned grade

• To make a suitable field surface to control flow of water

• To check soil erosion and • To provide for surface drainage.

Irrigated areas benefit greatly from land grading since the ground in its natural state is seldom suited for the efficient application of water

In low rainfall areas, land grading • Produces a smooth uniform land surface, • Reduces runoff and induces infiltration of

rain fall• Assures even moisture distribution.

On sloping ground, levelling eliminates small depressions, cuts and furrows which leads to concentration of runoff.

Proper land grading, coupled with surface drainage measures, reclaim unproductive poorly drained areas.

Phases of Land levelling operations 1. Rough grading2. Land levelling3. Land smoothing.

Criteria for land grading are influenced by 1.The characteristics of the soil profile, 2.Prevailing land slope, 3.Rainfall characteristics, 4.Cropping pattern, 5.Methods of irrigation, 6.Other special features of the site 7.The preferences of the farmer.

LAND CLEARINGPrior to making the land grading survey, it is advantageous to remove heavy vegetative growth from the land. Land clearing consists of removing some or all of the trees, bush, vegetation, trash and boulders and all roots.

A well-designed irrigation system delivers the required amount of water to all parts of the area to be irrigated at the required rate without damage to the soil or excessive loss of water.

The topography is a major factor in selecting • The method of irrigation • Estimating the number and kind of water

control structures needed• Determining the need for land levelling • The relative elevations of the source and the

area to be irrigated• The drainage outlets

Land levelling design methods1.Plane method2.Profile method 3.Plan inspection method4.Contour adjustment method

Plane method Procedure:1.Determine the centroid of the field2.Determine the average elevation of

the field3.Compute the slope of the plane of

best fit4.Compute the formation levels, cuts

and fills5.Determine the cut-fill ratio

Mark grid points in the field

Survey the elevations and draw contour map

Determining the centroid of the fieldThe centroid of a rectangular field is located at the point of intersection of its diagonals. The centroid of a triangular field is located at the intersection of the lines drawn from its corners to the midpoints of the opposite sides. To determine the centroid of irregular fields, the area is divided into rectangles and right-angled triangles. The centroid is located by computing moments about two reference lines at right angles o each other.

Determine the average elevation of the fieldAdding the elevations of all grid points in the field and dividing the sum by the number of point give the average elevation.

Locate the centroid and fix its elevation as mean of the field

Compute the slope of the plane of best fit. The slope of any line in the x or y direction on the plane which fits the natural ground surface, can be determined by the least squares method.

nDD

nHDDHS

/}){()(

/)})({()(22

in whichS=slope of line in a p1ane, dimensionless*D= distance from the reference line, m H= elevation of the grid point, mn = number of grid points

Compute the formation levels, cuts and fills: With the elevation of the centroid determined, the formation level of any point (the elevation which the point should attain after land grading operation) may be determined, using the computed or assumed values of Sx and Sy.

Mark the existing and computed elevation on grid points and compute cut/ fill

Computation of earthwork volumes of field : L² ∑ C² Vc = ----------------------

4(∑ C + ∑ F)

L² ∑ F² Vf = --------------------------

4 (∑ C + ∑ F)In which, Vc = Volume of cut, m³ Vf = Volume of fill, m³ L = Grid spacing, m ∑ C = Sum of cuts on four corners of a grid

square, m ∑ F = Sum of fills on four corners of a grid

square, m

PROFILE METHOD, Essentially it consists of a trail and error method of adjusting grades on plotted profiles until the irrigation criteria are met with and the earthwork balance is attained.

PLAN INSPECTION METHODThe grid point elevations are noted on the plan, and the design grade elevations are determined by inspection after the careful study of the topography. It is largely a trial and error procedure.

In selecting the elevations formation level the designer must simultaneously consider the down field slope, cross slope, earth work balance and haul distance. The desired cut fill ratio and volumes of earthwork are estimated from the summation of cuts and fills.

The grades are frequently adjusted to obtain favourable earthwork balance and to maintain the down field and cross slopes within safe limits.

CONTOUR ADJUSTMENT METHODThe contour adjustment method of land levelling designs consists of trial and error adjustments of the contour lines on a plan map. The method is specially adapted to the smoothening of steep lands that have to be irrigated.

A contour map is drawn and the proposed ground surface is shown on the same map by drawing new contour lines. The uniformity of downfield slope is controlled by the uniformity of the horizontal spacing between contours, and the cross slopes can be examined by scaling the distance between contours at right angles to the direction of irrigation.